As problogger.com says, blogger have more users than WordPress.com. Here in this post I’m not going to compare self hosted WordPress and Blogger. Self hosted WordPress is highly customizable but you can’t customize WordPress.com as you wish (as a free user).
You can’t customize the theme or CSS in WordPress.com. If you really wanna edit those CSS files then you need to pay and get the permission. Personally I think you can buy a hosting for that money. In the other hand blogger offer you the opportunity to customize the template as you wish. XML enabled blogger themes helps no-voice user to edit CSS and design the layout as they wish without paying a penny.
To avoid SPAM comments you may enable Random Letters protection, but most of the users around blogosphere found it more irritating and annoying. WordPress.com is helping you to hunt down SPAMS using a technology ASKIMET. It’s very effective and it’s almost catching SPAMS.
Earlier WordPress.com didn’t support that Visitors’ stat. It means you don’t know how many people visited your blog, where in Blogger you can use almost any third party stat counter in your blog. Now WordPress.com also offering a wonderful stat tool which you can use also in yourself hosted blog.
You can use your customized header both in Blogger and WordPress.com, but when you’re using WordPress.com make sure the theme you use can supports the customizable header. Some themes don’t support this feature.
Incase if you want to move your blog from Blogger to WordPress, then WordPress.com got wonderful tool that can help you to migrate your posts, author even categories, but Blogger doesn’t have such feature where you can migrate your posts from another Blogging platform.
It is said that Blogger offering unlimited storage for image storing where WordPress.com offering 3GB (not a bad size) space for your blog. I’m using blog for 2 years and my blog still has not reached even 100MB so I don’t think 3GB will be a problem. It’s more than enough.
An optional excerpt is another advantage in WordPress.com. You can show the real punching line in your home page and make user to read the full post. This feature is not available in Blogger.
Now both Blogger and WordPress.com offers widget, so it doesn’t make much difference among both of them.
If you’re an advanced programmer or a person who’d like to play around with codes then please go for Blogger.com. WordPress.com restricts you from editing theme, CSS codes. In the other hand if you really wanna play around with WordPress themes, code, CSS then go for a self hosted WordPress account or become a premium member by paying to WordPress.com.
In my personal opinion I think, if WordPress.com can allow embedding external scripts and allows editing the CSS and themes then it’ll be a much better choice than the Blogger. We can’t predict anything. May be in future WordPress.com will remove the restrictions.